The recent progress of duplicators and printers is remarkable, and these are being drastically improved not only in point of the cost thereof but also the performance thereof such as the recording quality, the recording speed and others. Accordingly, at present, printed materials such as posters, catalogs, pamphlets and others that have been heretofore produced by the use of printing machines have become produced more simply and more rapidly by the use of duplicators or printers while maintaining the recording quality as before. Recording with such duplicators or printers is especially favorable in quickly producing a variety of but a small number of printed copies.
Of various types of duplicators and printers, electrophotographic duplicators and printers, especially those driven by semiconductor laser are excellent in point of high resolution and rapid recording capability. Electrophotographic system enhancement technology toward color image formation has some problems in that the apparatus must be large-sized as requiring plural developing devices and that the color tone of the image formed is slightly poor as pigment is used for the toner colorants. However, these problems are being solved by downsizing the apparatus and by improving the materials for toners, recording sheets, etc.
Regarding the improvement of recording sheets, an electrophotographic recording sheet laminated with a synthetic resin film having a coating layer on at least one surface of a substrate layer made of a water-resistant paper material (Patent Reference 1) has been invented and put into practical use, which is more excellent in the recording quality, the fixability, the water resistance and the durability than conventional electrophotographic recording sheets made of pulp paper.
However, the conventional electrophotographic recording sheet laminated with a synthetic resin film having a coating layer thereon, on at least one surface of the substrate layer made of a water-resistant paper material is, though excellent in the recording quality, the fixability, the water resistance and the durability, still problematic in that, in continuous printing thereon, the static charge given to the recoding sheet inside the printer could not be well discharged, and therefore, when the printed sheets are left as piled up after printing thereon, then they may often block together and (at their edges) may be difficult to pile up uniformly (at their edges). Accordingly, when the printed materials are further worked by cutting or blanking, then they may have another drawback in that their size may differ and their pattern position may fluctuate. As recording sheets that are prevented from being statically charged after printing and are therefore prevented from blocking together, there are proposed those coated with an antistatic agent (see Patent References 2 to 4); and those in which an antistatic agent is added to the toner-receiving coating layer (see Patent References 5 to 7).
However, the proposals heretofore made in Patent References 2 to 7 are for those coated with a water-soluble antistatic agent as a simple substance thereof, and those for which a water-soluble antistatic agent is added to the water-base coating material. Accordingly, these are still problematic in that, though their supports are resistant to water by themselves, the coating layers are still poorly resistant to water, and therefore they are not durable in practical use such as outdoor use, etc. These coating layers are too hydrophilic, and therefore, when the printed materials are dipped in water for 24 hours and then the printed surfaces are rubbed strongly, then the toner is readily peeled off along with the coating layer, or that is, the coating layer is incomplete.
On the other hand, an electrophotographic recording sheet that comprises a moisture-absorbing (ambient humidity-dependent) antistatic agent has some drawbacks in that the printed sheet could not stably exhibit the antistatic capability owing to the reduction in the relative humidity around the surface of the sheet that is overheated with the toner fixing unit of the electrophotographic recording system used and that, in continuous printing in low-humidity environments in the winter season or the like, the printed sheets may often block together.
Patent Reference 1: JP-A 2002-091049
Patent Reference 2: JP-A 8-297375
Patent Reference 3: JP-A 2000-131870
Patent Reference 4: JP-A 2004-083689
Patent Reference 5: JP-A 8-011263
Patent Reference 6: JP-A 2004-083689
Patent Reference 7: JP-A 2005-271396